refactor: move from io/ioutil to io and os package

The io/ioutil package has been deprecated as of Go 1.16, see
https://golang.org/doc/go1.16#ioutil. This commit replaces the existing
io/ioutil functions with their new definitions in io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

cmd/ctr/app/main.go

@@ -18,7 +18,7 @@ package app
 
 import (
 	"fmt"
-	"io/ioutil"
+	"io"
 
 	"github.com/containerd/containerd/cmd/ctr/commands/containers"
 	"github.com/containerd/containerd/cmd/ctr/commands/content"
@@ -46,7 +46,7 @@ var extraCmds = []cli.Command{}
 
 func init() {
 	// Discard grpc logs so that they don't mess with our stdio
-	grpclog.SetLoggerV2(grpclog.NewLoggerV2(ioutil.Discard, ioutil.Discard, ioutil.Discard))
+	grpclog.SetLoggerV2(grpclog.NewLoggerV2(io.Discard, io.Discard, io.Discard))
 
 	cli.VersionPrinter = func(c *cli.Context) {
 		fmt.Println(c.App.Name, version.Package, c.App.Version)